Binocular contrast, stereopsis, and rivalry: Toward a dynamical synthesis.

It is well known that small orientation differences between two monocular gratings fuse to generate a stereoscopic perception of tilt, while large differences trigger binocular rivalry. In addition, unequal monocular contrasts combine nonlinearly to generate binocular contrast. A nonlinear neural model is developed here to account for binocular contrast, fusion at small orientation differences, and rivalry at large differences. The model also accounts for hysteresis in the transition between fusion and rivalry. Finally, the model predicts that interocular contrast differences between fusible gratings will produce a reduced tilt percept, and experiments reported here support this. Key to the model is the presence of two classes of inhibitory interneurons: one operating on similar orientations to normalize interocular contrast (IN), and one operating across large orientation differences to generate rivalry (IR). Critically, the IN neurons switch off the IR neurons driven by the other eye, thus permitting fusion of binocular plaids.